Computer systems typically include devices that communicate using a bus. A bus is one or more signal paths, or transmission lines, that carry signals, from a sending device to a receiving device. Both the sending and receiving devices may be integrated circuits. Buses are also used to carry signal from sending and receiving devices that are located on the same integrated circuit.
The speed and integrity of the transmission of signals over the bus is often of critical importance to the operation of the devices and the overall system. Signal reflection can interfere with the signal transmissions. Signal reflections are created when there are impedance mismatches, or discontinuities in the system. For example, signal reflections can be created when a signal is transmitted over a transmission line that interfaces with a component that has an impedance which differs from the characteristic impedance of the transmission line. Reflected signals are problematic because they can interfere with transmitted signals and affect the integrity of the transmitted signal. One method to deal with this issue is to delay sending or receiving subsequent transmitted signals until the reflections from the previous transmitted signal have subsided. The disadvantage to this approach is that it limits the speed and the efficiency of the devices and the overall system.
Another approach is to employ transmission line terminators. A terminator is a dissipative load, typically a resistor, located at or near the end of a transmission line. Generally, a terminator is selected having an impedance that is matched to the characteristic impedance of the transmission line.
Transmission line termination may be done on or off chip. When “off-chip” termination is used, the signal is terminated by termination circuitry that is located outside of or “off” the integrated circuit receiving the transmitted signal. When “on-chip” termination is employed, the signal is terminated by termination circuitry which is located “on” or within the integrated circuit.
FIG. 1 illustrates an off-chip termination scheme. A first integrated circuit 100, which is also a sending device, transmits a signal through a transmission line 125. A termination circuit 130 is coupled to transmission line 125 near a second integrated circuit 140, which is also a receiving device. Since the termination circuit 130 is outside the second integrated circuit 140, the termination scheme is considered off-chip termination. Since the termination circuit 130 is off-chip and outside the second integrated circuit 140, power associated with the termination of the signal is dissipated by the termination circuit 130 rather than within the second integrated circuit 140. Thus, the power requirements and the performance of second integrated circuit 140 are not affected by the power dissipated within the termination circuit 130.
By the inherent nature of off-chip termination, there is a signal path between the termination circuit 130 and the receiving device 140. The signal path between the termination circuit 130 and the receiving device acts as an impedance discontinuity, or like a secondary transmission line with its own characteristic impedance. This impedance discontinuity can degrade the integrity of the signal received by the receiving device. Accordingly, a solution to reduce or eliminate this so-called secondary transmission line is needed.
An on-chip termination scheme can effectively eliminate the secondary transmission line problem by locating the termination circuitry within the integrated circuit having the receiving device. FIG. 2 illustrates an on-chip termination scheme. First integrated circuit 200, which is also a sending device, transmits a signal through transmission line 225 to termination circuit 230 located within a receiving device on a second integrated circuit 240. Termination circuit 230 is incorporated within the circuitry of the second integrated circuit 240. Since the termination circuit 230 is incorporated within the second integrated circuit 240, signal reflections are minimized thus ensuring a signal having good integrity is available to the receiving device of second integrated circuit 240. Similar to the termination circuit in the off-chip termination scheme, termination circuit 230 is designed to prevent a reflected signal from being sent back to the first integrated circuit 200. Furthermore, because the termination circuit 230 is within the second integrated circuit 240, there is no secondary transmission line. Accordingly, on-chip termination generally allows signal transmission rates to be increased without a decrease in signal integrity due to the effects of the secondary transmission line. As a result, it is usually possible to use a higher signal transmission rate in systems using on-chip termination rather than off-chip termination.
Power dissipation associated with on-chip termination schemes can however, be problematic. When signals are terminated on chip, power is dissipated by the termination circuit within the integrated circuit. Power is dissipated when the voltage level of the incoming signal differs from the voltage level of the termination voltage. The additional power dissipation increases the operating temperature of the integrated circuit. This can cause a reduction in the operating speed and overall performance of the integrated circuit. Furthermore, additional power dissipation in the integrated circuit increases the power supply requirements for the integrated circuit. A termination scheme that has reduced, or is free of, power dissipation and secondary transmission line issues is desirable.